Twistlocks are devices that are used in transport, and are commonly used with shipping containers conforming to ISO regulation (International Standards Organisation). Such twistlocks are predominantly used in shipping where the twistlocks interlock between corner castings of two adjacent containers, or in shipping or rail freight where containers are fixed to fixed castings provided on rail carriages or the deck of a ship. Other uses also exist. Where the term corner casting or container casting are used in this specification they are intended to refer to castings of the type found on corners of shipping container, or fixed on a surface for interconnection with such containers.
Twistlock have traditionally taken the form of a housing, through which a shaft extends joined to or formed with a respective cone on either end of the shaft and positioned on opposing sides of the housing. The cones are somewhat elongate to fit through the rectangular slot in the corner casting, and to lock within the corner casting on rotation to a position transverse to the slot. Where both cones are locked within a respective casting the two adjacent castings are locked together.
Various advances have been made in twistlocks and some of these have been directed at improving the inefficiencies of using manual twistlock. The operation of a manual twistlock requires that a longshoreman physically mounts the container, unlocks all twistlocks so that the container can be lifted off. In shipping the longshoreman then has to mount the container again, remove the twistlocks, and then unlock the container from the ground before the bottom container is removed. For loading the reverse is the case. Several proposals have been made for semi-automatic twistlocks to minimise the effort required in loading and unloading and several versions are commercially in use. These semi-automatic twistlocks facilitate loading, in that the cones are biased, for example, by the shafts being spring loaded to a locked position. The lower cones of the twistlock are placed in position either on the bottom container or the fixed casting. The container is lowered onto the cones, which are so formed that the downwards vertical force impart a rotational force on the shaft which forces the cone into a position allowing the cone to pass through a slot in the casting to the inside of the hollow casting, whereupon the cone snaps into the locked position within the container casting by reason of the torsional resistance conferred by the spring.
There is still however a considerable amount of work needed in the unloading with such semi-automatic twistlocks because a lever or other device needs to be manually actuated to keep the cone in an unlocked position, as a result each twistlock must be separately handled. It is desirable to provide an automatic twistlock that can be unloaded in a manner similar to that by which it is loaded.
One avenue for making the twistlocks automatic lies in the shape of the cones of the twistlock. Taking the top cone of a semi-automatic twistlock, the upper surface is bevelled at an angle such that when the cone is forced against the edges of the elongate slot of the corner casting, the shaft turns from a locked position to an unlocked position (i.e. aligned with the elongate slot) against a spring and on entering the corenr casting snaps into the locked position within the casting. Therefore, should the lower surface be bevelled appropriately then a similar result might be achieved. The shape of the lower surface must be trialled carefully because if the shape facilitates rotation of the cone too easily then there will be a tendency for the twistlock not to lock the container sufficiently well into place during transport, especially where two such cones exist on the same twistlock then the tendency of the twistlock to dislodge is quite high.
There has been a proposal for a twistlock that is automatic, to the extent that the locking and unlocking of an upper container is achieved by the application of downward and upward vertical forces respectively. The lower cone of the proposed twistlock has an upper surface that is substantially flat and is intended to be locked into position on a lower casting. This twistlock is suggested for use where only one container or layer of containers is to be loaded onto a fixed casting, such as for rail transport. Because the forces on containers loaded for shipping also include large vertical components these twistlocks are not appropriate for shipping activities.
The above proposal however has several disadvantages, the primary disadvantage arises as a result of rough and somewhat careless handling and the high loads that are exerted on the twistlock in view of the weight of material carded within containers. Where the container is lifted by crane or gantry onto the fixed corner castings with twistlocks in place, there is usually some sideways movement; it is rare for the corner castings of the container to engage the top cone of the twistlock with only a vertical motion. As a result it is highly likely that the twistlocks may be tilted with respect to the fixed corner casting on loading. As a result either the twistlock may become jammed in the corner casting, or the lower cone can be jammed within the lower casting, so the that top cone cannot rotate to align with the elongate slot of the casting on the container. The twistlock then needs to be physically removed, and that may not always be easy, and may result in damage to the twistlock or the corner casting.
Additionally if the lower cone of the twistlock is twisted as a result of the loading and becomes aligned with the elongate slot of the corner casting, it is possible that the twistlock may be flipped out of the lower casting.
It is an object of the present invention to provide an automatic twistlock arrangement that obviates or minimises any one of the above problems or at least provides the public with a useful choice.